I. Field of Invention
This invention relates to a trolley wheel in general, and in particular, a unique plastic wheel assembly particularly adapted for utilization in sanitary environments.
II. Description of the Prior Art
Overhead conveyor trolleys are used in numerous applications in manufacturing and fabrication plants as well as in food processing plants or other types of installations such as hospitals or the like where sanitation and noncorrosive properties are critical. In many food processing installations, food or other sanitation sensitive items are carried by the overhead conveyor and the conveyors are many times passed through a steam bath or the like each time the conveyor makes a circuit. In many cases, the conveyor is apt to be exposed to elements such as heat and moisture or detergents which would quickly cause rust or other types of deterioration if conventional conveyor assemblies were utilized. Another type of deterioration in such a system would be loss of lubrication when the conveyor is passed through the cleaning process.
The conventional overhead conveyor wheel is a ball bearing supported conveyor which utilizes a trolley wheel riding in or on a track and is connected to a bracket which has a chain engaging portion. The chain is driven by a conventional power means and the load is suspended from the bracket. Conventional prior art conveyors utilize metal wheels which are subject to corrosion and the bearings require frequent lubrication. If these type of prior art conveyors are used in a sanitary environment, that is subjected to steam baths etc. the lubrication is destroyed and corrosion problems are encountered such as the wheels tend to wear severely at the bearing. This results in problems of the system "binding up" causing rapid flaking or peeling of metal material which results from metal-on-metal contact. Thus the system deteriorates quickly with adverse side effects. On the other hand if additional lubrication is applied to overcome this problem, the system is not "sanitary" since the lubrication drops into the articles (such as food) being conveyed. While plated trolleys work for a while, the plating eventually wears. All stainless steel parts on the other hand are too expensive.
In order to overcome many of the disadvantages of the conventional metal trolley, it has been considered to provide a plastic trolley wheel which will withstand deterioration from exposure to heat and moisture. Ideally, utilization of plastic would also provide a quieter system which is desirable in most installations. Plastic wheels of a sort, of course, have been known in such installations as a suspended shower door movable in its own plane along plastic rollers connected to the top and movable in a track. One type is illustrated in Langstroth, U.S. Pat. No. 3,469,892, which utilizes a plastic wheel and bearing assembly, the peripheral portion of the wheel or opening being formed to provide an outer race for the ball bearing. These types of prior art proposals, of course, do not take into consideration situations where the loads are significant nor are they really directed toward situations where sanitation itself is that significant.
Attempts have been made to provide a load bearing plastic trolley assembly in situations where the loading is more substantial. A more recent representation of such teaching is that illustrated in Dawson, U.S. Pat. No. 3,787,925. These types of installations however have not really operated as well as expected. A particular difficulty is that these types of plastic wheels do not utilize a rolling element bearing mechanism. While a very high density wheel material is utilized, so that it is load supporting, the utilization of a journal bearing construction has continued to create a variety of problems. To begin with, in a journal bearing arrangement where the wheel rides on a journal or freely rotates about a stud, high start up torque results. Further, the speed of operation is limited since as the speed builds up, the heat builds up thereby increasing the wear. Such types of wheels have also experienced flat spots internally near the journal and when this occurs, not only does the wheel tend to bind up, resulting flat spots will occur on the outer surface portion of the wheel due to the resultant eccentric rotation. As soon as flats are experienced, increased flaking of the material is experienced which is extremely disadvantageous in sanitary environments utilizing food or the like. A further disadvantage has been a tendency to squeak in operation requiring mineral oil lubrication which further deteriorates rapidly in installations utilizing a process for cleaning the conveyor system. The journal bearing system is not self-cleaning and deposits caused from flaking or ingestion of foreign material decreases the efficiency and operation.
The prior art to date has found that high density ultra high molecular weight polyethylene (UHMW) has proven to be most desirable where plastic trolley wheels are utilized. This is particularly true in sanitary or non-corrosive applications.
If the higher density range is not utilized, then the wear, particularly at the bearing area is too great. UHMW polyethylene in the higher density range however, is considerably more expensive than that in the lower density range and is not subject to injection molding. Thus, not only is the material cost higher, it requires machining which greatly adds to the overall cost of the trolley wheel assembly.
Yet another drawback to journal bearing plastic wheel trolleys is the high start up torque involved. This requires not only increased wear but also a power supply larger than desired. Thus, although it has been recognized that there are advantages to utilizing a plastic trolley wheel in a load bearing conveyor assembly, there is a need in this art for an improved arrangement which permits high speed operation at low torque start up utilizing a conveyor wheel not subject to rust and which truly does not need periodic lubrication and in fact is self-cleaning. In addition, there is a need to provide a plastic trolley wheel arrangement providing longer service life at an economically feasible price.